This invention relates to polarizing glasses. More particularly, the invention relates to polarizing glasses and polarizing articles such as optical isolators made from such glasses.
A polarizing effect can be obtained in glasses containing silver halide, copper halide, or copper-cadmium halide crystals. These crystals can be precipitated in aluminosilicate glasses having compositions containing suitable amounts of an indicated metal and a halogen other than fluorine.
The polarizing effect is generated in these crystal-containing glasses by stretching the glass, and then exposing its surface to a reducing atmosphere. The glass is placed under stress at a temperature above the glass annealing temperature to elongate the glass, thereby elongating and orienting the crystals. The elongated article is then exposed to a reducing atmosphere at a temperature above 250xc2x0 C., but not over 25xc2x0 C. above the glass annealing point. This develops a surface layer in which at least a portion of the halide crystals are reduced to elemental silver or copper (hereafter xe2x80x9cmetalxe2x80x9d).
The growth of halide particles cannot occur at temperatures below the strain point of the glass because the viscosity of the glass is too high. Higher temperatures, above the annealing point, are preferred for crystal precipitation. Where physical support is provided for the glass body, temperatures up to 50xc2x0 C. above the softening point of the glass can be employed. A process for the production of polarizing glass is described in detail in U.S. Pat. No. 4,479,819.
Polarizing glasses of the type described above are used to make polarizing elements that are used in equipment including but not limited to optical communication equipment, optical recording equipment, optical sensors, and optical interferometers. Optical isolators, which are used in optical communication systems, include a first polarizer, a Faraday rotator, and a second polarizer arranged on an optical axis in a holder. Two major uses of optical isolators are in optical transmission systems and in fiber-optic amplifiers. Insertion loss of an optical isolator is an important consideration in the manufacture of an optical isolator, and insertion loss should be kept to a level less than about 0.5 dB.
Insertion loss can generally be attributed to three factors: surface loss, absorption loss and coupling loss. Surface loss is the loss between the various surfaces of the components that make up an optical isolator, and differing refractive indices between components contribute to surface loss. The refractive index of the polarizing glass described in U.S. Pat. No. 4,479,819 is 1.508 or less. The refractive index of the Faraday rotator is typically around 2.4, and this large difference in refractive index contributes to the surface loss in an isolator component. Even though antireflection (AR) coatings can reduce surface loss, it would be desirable to provide other ways to reduce surface loss. Coupling loss is typically between about 0.15 and 0.25 dB. It would be desirable to provide a polarizer made from a polarizing glass that having reduced coupling and surface losses when compared with conventional polarizers.
One embodiment of the present invention relates to a polarizing glass article that is phase-separated by precipitating silver halide, copper halide, or copper-cadmium halide crystals and that contains silver, copper, or copper cadmium metal particles having an aspect ratio of at least 2:1. According to this embodiment, the article has a refractive index greater than 1.55. In other embodiments, the article has a refractive index greater than 1.6. In preferred embodiments, the article has a refractive index greater than 1.65.
Another embodiment of the invention pertains to optical isolators including polarizing glass articles made in accordance with the present invention. One advantage of optical isolators using polarizers in accordance with the present invention is that isolators exhibit low coupling loss. For example, according to one embodiment, an optical polarizer having a refractive index greater than 1.6 and a thickness of 50 microns disposed between two single mode fibers has a coupling loss less than 0.35 dB.
Another embodiment of the invention relates to a method of manufacturing a polarizing glass article. The method includes the steps of melting a batch of glass having a refractive index greater than 1.6 and containing a source of silver, copper or copper and cadmium and at least one halogen, cooling and shaping the melt into a glass article, and subjecting the glass article to an elevated temperature for a period of time sufficient to generate and precipitate silver halide, copper halide or copper-cadmium halide crystals in the glass. The method also includes the steps of elongating the glass article under stress at a temperature above the annealing point of the glass to elongate the crystals in the direction of the stress and exposing the elongated glass article to a reducing atmosphere at an elevated temperature to initiate reduction to silver or copper metal. In a preferred embodiment, the glass article has a refractive index greater than 1.6, and in a highly preferred embodiment, the glass article has a refractive index greater than 1.65. In certain embodiments, the temperature for precipitating silver halide, copper halide or copper-cadmium halide crystals in the glass is at least about 50xc2x0 C. above the softening point of the glass and the time sufficient for precipitating the crystals is between two hours and four hours. In other embodiments, the temperature at which the glass article is elongated under stress is at least about 50xc2x0 C. below the softening point of the glass. In other embodiments, the temperature at which the glass article is exposed to a reducing atmosphere is greater than about 250xc2x0 C.
Additional advantages of the invention will be set forth in the following detailed description. It is to be understood that both the foregoing general description and the following detailed description are exemplary and are intended to provide further explanation of the invention as claimed.